The present invention generally relates to fabrication of semiconductor devices and more particularly to a process for fabricating a semiconductor device including therein a semi-insulating compound semiconductor layer of a group III-V compound semiconductor material.
In the high speed compound semiconductor integrated circuits that utilize the group III-V compound semiconductor materials such as MESFETs, HEMTs or HBTs, a high purity semi-insulating semiconductor material having high resistivity is needed for the device isolation. In the optical semiconductor devices such as laser diodes and the like, too, such a semi-insulating, high resistivity semiconductor material is used for current confinement.
Conventionally, such semi-insulating semiconductor layers have been formed by introducing elements such as chromium or titanium that form the deep impurity level in the semiconductor material at the time of growth of the semiconductor layer. Particularly, such chromium or titanium is called "deep acceptor" and causes the pinning of the Fermi level generally at the midpoint of the conduction band and the valence band.
FIG. 1 shows the principle of pinning the Fermi level. Referring to FIG. 1, the vertical axis at the left shows the concentration of carriers (p, n) as well as the concentration of donors (N.sub.D) and acceptors (N.sub.A) including their ionized states (N.sub.D.sup.+, N.sub.A.sup.-). On the other hand, the vertical axis at the right shows the resistivity of the semiconductor material. Further, the horizontal axis shows the energy. It should be noted that Ev represents the valence band energy, Ec represents the conduction band energy, E.sub.A represents the shallow acceptor level, Ed represents the deep donor level, E.sub.F represents the Fermi level in the pinned or compensated state, and E.sub.F ' represents the Fermi level in the uncompensated state. Thus, this diagram shows the pinning the Fermi level in the semi-conductor material that includes the shallow acceptor by doping of the deep donor.
Referring to FIG. 1, the thin continuous line designated as "p" represents the concentration of the holes formed in the valence band in the thermal equilibrium state while the thick line designated as "p+N.sup.+.sub.D " represents the concentration sum of the holes and the ionized donors in the thermal equilibrium state. As is well known, the concentration of holes p and the concentration of the ionized donors N.sub.D.sup.+ are determined by the condition of thermal equilibrium as: EQU p=Nv exp[-(E.sub.F -Ev)/kT], (1)
and EQU N.sub.D.sup.+ =N.sub.D exp[1+g.sub.D exp(E.sub.F -Ed)/kT].sup.-1,(2)
where Nv represents the effective density of state of holes, g.sub.D represents the degeneracy factor and k represents the Boltzman's constant.
Thus, Eq.(1) determines the thin continuous line for the holes designated as "p" in FIG. 1 as a function of E.sub.F, and Eqs.(1) and (2) define the thick continuous line designated as "p+N.sub.D.sup.+ " also as a function of E.sub.F.
Similarly, the concentration level of electrons, n, and the concentration level of the ionized acceptors, N.sub.A.sup.-, are represented as EQU n=Nc exp[-(Ec-E.sub.F)/kT, (3)
and EQU N.sub.A.sup.- =N.sub.A exp[1+g.sup.-1.sub.A exp(E.sub.A -E.sub.F)/kT].sup.-1( 4)
Thus, Eq.(3) determines the thin continuous line for the electrons designated "n" in FIG. 1 as a function of E.sub.F, while Eqs.(3) and (4) define the thick continuous line designated "n+N.sub.A.sup.- " also as a function of E.sub.F.
Further, there holds a condition of electroneutrality as: EQU p+N.sub.D.sup.+ =n+N.sub.A.sup.-. (5)
Thus, by solving Eqs.(1) through (5) simultaneously, the Fermi level E.sub.F and the corresponding carrier concentration, p and n, are determined simultaneously. The carrier concentration, in turn, determines the resistivity of the semiconductor material. In FIG. 1, the resistivity thus determined is plotted as a function of E.sub.F.
Considering now the case of there is no deep donor, the quantity N.sub.D and thus the quantity N.sup.+.sub.D are all zero or in the negligible order. Thereby, the condition (5) is written as EQU p=n+N.sub.A.sup.-. (5')
When this is the case, the Fermi level E.sub.F is determined to have a value E.sub.F ' in correspondence to a point A where the line "n+N.sub.A.sup.- " intersects the line "p". Given the Fermi level as such, the resistivity of the semiconductor material is determined to have a value of about 20 .OMEGA.cm.
On the other hand, when a deep donor is introduced with a suitable concentration level, the Fermi level E.sub.F is given by a point B where the line "p+N.sub.D.sup.+ " and the line "n+N.sup.-.sub.A " intersect each other. In this case, the Fermi level E.sub.F is pinned generally at the midpoint of the valence band Ev and the conduction band Ec, and the resistivity of the semiconductor material becomes maximum in correspondence to the point B' because of the minimum concentration level of the carriers.
The same argument holds true also for the case where the semiconductor material includes the shallow donors and deep acceptors. Thus, by doping the semiconductor material suitably by the deep donors or deep acceptors, it is possible to make the semiconductor material, particularly the compound semiconductor material, to have a large resistivity. Such a semi-insulating semiconductor material is used widely for the insulating layers in the compound semiconductor devices in which no convenient oxide insulation is available. Conventionally, elements such as chromium, titanium, manganese, iron, cobalt, nickel, copper and the like, are used for this purpose.
When such a deep impurities are introduced into the semiconductor layers, on the other hand, there arises a problem in that the impurities tend to diffuse into the active region of semiconductor devices and cause various problems in the operation of the device. For example, when the impurity elements are diffused into the active region of laser diode, such an impurity acts as the anti-luminescent center that traps carriers and the efficiency of laser oscillation is significantly deteriorated. On the other hand, when the impurity elements enter into the active region of fast-speed devices such as MESFET or HEMT, the mobility of carriers is decreased by the scattering caused by the impurities.
Conventionally, it is known that there exists a type of defect known as EL2 that acts as the deep donor in the compound semiconductor material such as GaAs. This defect, EL2, is formed by the As atom occupying the Ga site and thus does not include atoms that are exotic to the system of GaAs or a solid solution system thereof. Thus, when this EL2 could be used for the deep donor, the foregoing problems caused by the impurity elements are all eliminated.
Unfortunately, the concentration level of EL2 has conventionally not been controlled as desired, as EL2 is not formed as a result of the controlled doping but as a result of the growth process itself. Usually, the concentration of EL2 is in the order of 10.sup.14 cm.sup.-3 and does not change with the growth condition such as the temperature. With such a low and uncontrolled level of concentration, EL2 cannot be used for the deep donor for pinning the Fermi level at the center of the band gap.